Muscle excercise devices and associated methods

ABSTRACT

A muscle exercise device can include a crank having an axis of rotation. A first arm can be connected to the crank so that a downward external force applied to the distal end of the first arm causes rotation of the crank. The devices of the present invention can further include a second arm to which a counterweight is connected. Optionally, an assist mechanism other than a counterweight can be used to provide upward movement of the first arm with minimal application of force by a subject. A retrofit kit can be provided for modifying an existing device such as an ergometer or road bicycle. The retrofit kit can include a counterweight and a fastener for attaching to the device to the existing device. The devices of the present invention allow for increased exercise capacity, isolation of muscle groups, and biomechanical forces which closely approximate comparable paired limb motion.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Nos.60/794,662, filed Apr. 24, 2006 and U.S. Provisional Application Nos.60/875,216, filed Dec. 15, 2006, each of which are incorporated hereinby reference.

BACKGROUND AND SUMMARY OF THE INVENTION

Ventilatory patterns change during exercise in response to the increasedoxygen demands of muscles and other tissues. Such changes are transient,and largely disappear once the metabolism slows down and any oxygen debtis repaid. Persisting with a regimen of moderate exercise, however, cancause more lasting physiological changes, including an increasedcapacity for oxygen metabolism in muscle groups exercised and anenhanced ability of the circulatory system to deliver oxygen to thosemuscle groups. Increased respiratory capacity of muscles from exerciseis a direct response to increased rate of oxygen delivery to thosemuscles. During exercise the amount of oxygen taken up by the blood fromthe respiratory system must be distributed among various muscle groupsaccording to their level of activity. Therefore the degree to whichheavy activity can be focused to fewer muscle groups at a given maximumrate of oxygen uptake (VO₂max), those muscles will experience a higherrate of oxygen delivery.

A number of applications can benefit from more effective and efficientmodes of aerobic exercise. One is rehabilitative exercise for patientssuffering the effects of congestive heart failure, vascular disease, andobstructive pulmonary disease. Often these patients have severelyreduced exercise capacity and exhibit low VO₂max values. Such low oxygendelivery cannot allow paired-limb exercise of sufficient intensity toprovide an up-regulation of muscle aerobic capacity. This isparticularly important to the prognosis of chronic heart failurepatients, whose chances of surviving a heart transplant can be greatlyincreased by an increase in ventilatory threshold (an indicator ofmetabolic stress in skeletal muscle). Another application for whichintensity of exercise is important is the training of enduranceathletes. Exercise that produces greater gains in single leg ventilatorythreshold would allow endurance athletes to perform for prolongedperiods at close to VO₂max without experiencing metabolic stress inskeletal muscle.

Applications such as these, point to a need for methods of exercise thatisolate muscle groups such as those in a single limb. Also needed aredevices that can facilitate such exercise. Many aerobic exercise methodsare useful for concentrating a subject's exertions to certain musclegroups. Among the most effective methods for generating sustainedaerobic activity are those that approximate locomotion. However suchmethods and the devices that implement them typically involvepaired-limb activity. In this type of exercise the total oxygen uptake,less that required for other body processes, is more or less equallydivided between two limbs. Therefore the maximum delivery rateexperienced by each limb can only approach one-half of VO₂max. Toprovide maximum gains in respiratory capacity, it has been recognized bythe present inventors that it would be preferable to exercise eachindividual limb in isolation. In this way, each limb can receive oxygenat a rate much closer to VO₂max, which can result in a greater adaptiveresponse by the muscles of that limb.

Accordingly, the present invention provides a cardiovascular and muscleexercise device including a crank having an axis of rotation. A firstarm can be connected to the crank and can be configured so that adownward external force applied to the distal end of the first armcauses rotation of the crank. The devices of the present invention canfurther include a second arm to which a counterweight is connected andconfigured such that the force applied by the counterweight causesrotation of the crank. In accordance with a more detailed aspect of thepresent invention, the device can include a brake. Another aspect of theinvention includes a shield configured to prevent the counterweight fromstriking the subject during exercise.

In one embodiment of the invention the device can be a bicycle traineror a retrofit bicycle trainer. In another aspect of the presentinvention a retrofit kit can be provided for modifying an existingdevice in order to accomplish the purposes of the present invention. Theretrofit kit can include a counterweight and a fastener for attaching tothe device.

The invention also provides a method of exercising musculatureassociated with a single limb, comprising the steps of applying adownward force to an arm operatively connected to a crank so that thearm moves from an upper position to a lower position. The crank can thenbe allowed to continue rotating so that the arm returns to the upperposition. The return portion of the cycle is arranged such that thereturn movement is assisted by a counterweight.

Additional features and advantages of the invention will be apparentfrom the detailed description which follows, which illustrates, by wayof example, features of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of power applied to a crank arm versus crank angleduring a cycle of rotation for paired-limb and single-leg pedaling on aconventional exercise bicycle and single-leg pedaling in which a 30 lb.counterweight is attached to the opposite crank-arm in accordance withan embodiment of the present invention.

FIG. 2 shows the oxygen consumption as a function of power in a humansubject on an exercise bicycle during conventional paired-limb pedalingand single-leg pedaling assisted by a 30-lb counterweight in accordancewith an embodiment of the present invention.

DETAILED DESCRIPTION

Reference will now be made to exemplary embodiments of the presentinvention, and specific language will be used herein to describe thesame. It will nevertheless be understood that no limitation of the scopeof the invention is thereby intended. Alterations and furthermodifications of the inventive features illustrated herein, andadditional applications of the principles of the inventions asillustrated herein, which would occur to one skilled in the relevant artand having possession of this disclosure, are to be considered withinthe scope of the invention.

A. Definitions

The singular forms “a,” “an,” and “the” include plural referents unlessthe context clearly dictates otherwise.

As used herein, “subject” refers to a human or an animal, such as amammal, that can utilize and benefit from the devices and methodsdisclosed. Most often, the subject will be a human.

The term “axle” refers to a shaft made of rigid material, having an axisof rotation that is parallel to the shaft length and passes through itscenter and each end. Generally, the axle is positioned in a housing thatsupports the shaft and allows it to rotate on the axis of rotation. Anaxle can be a solid body, or can be partially or completely hollow.

As used herein, the term “limb” refers to an appendage that extends fromthe axial skeleton of a vertebrate, and incorporates an articulatedskeletal structure with musculature for moving the appendage through atleast one degree of freedom. When used with regard to limbs, the term“extension” refers to a movement or sequence of movements of the limbthat tends to straighten it or to move a portion of the limb away fromthe axial skeleton, while the term “flexion” refers to a movement orsequence of movements that tend to bend the limb or bring it closer tothe axial skeleton.

As used herein, the term “power curve” is a graph of power (usually inwatts) versus rotational angle (i.e. 0° through 360°) as applied to anaxle of an exercise device.

The term “about” when referring to a numerical value or range isintended to encompass the values resulting from experimental error thatcan occur when taking measurements and explicitly includes the exactnumerical value as if such were independently recited, unless otherwisenoted.

As used herein, a plurality of items, structural elements, and/ormaterials can be presented in a common list for convenience. However,these lists should be construed as though each member of the list isindividually identified as a separate and unique member. Thus, noindividual member of such list should be construed as a de factoequivalent of any other member of the same list solely based on theirpresentation in a common group without indications to the contrary.

Weights, distances, and other numerical data can be presented herein ina range format. It is to be understood that such range format is usedmerely for convenience and brevity and should be interpreted flexibly toinclude not only the numerical values explicitly recited as the limitsof the range, but also to include all the individual numerical values orsub-ranges encompassed within that range as if each numerical value andsub-range is explicitly recited.

B. Invention

The present invention provides an exercise device with which a subjectcan exercise primarily a single muscle group, such as the musculatureassociated with a single limb. Such a device can be utilized with onelimb more efficiently and effectively than a conventional ergometerdesigned for tandem two-limb exercise. A device in accordance with thepresent invention can comprise a crank to which two arms are connected,where a first arm can be used to transmit to the crank an externalforce, such as can be applied to the device by the subject, while asecond arm can be used to transmit to the crank a force supplied from anassist mechanism, and where both arms can transmit forces sufficient tocause the crank to rotate.

The crank of the present invention can include a central axle. The axlecan preferably be substantially cylindrical, though othercross-sectional shapes can be suitable such as, but not limited to,square, triangular, or star-shaped. Any cross-sectional shape that willallow the axle to rotate in its housing can be used in accordance withthis invention. In a particular aspect, the axle can include a flywheelpositioned at some point along the axle's length and concentric to theaxle. The flywheel will preferably be substantially circular, thoughother shapes that do not interfere with the ability of the flywheel andaxle to rotate can be used. An outer edge of the flywheel can be smooth,or it can include notches or protuberances by which the flywheel can beengaged by other devices such as belts, chains, sprockets, or latches.One specific example can include a flywheel having a toothed outer edgethat engages a chain, which in turn communicates rotation of theflywheel to another component such as a wheel or generator. Further, thecrank can have various contours which provide structural and/oraesthetic benefits.

The device can also comprise a first arm that is connected to the crankat a point along its length, preferably near an end thereof, and canextend from the crank in a direction that is substantially perpendicularto the axis of rotation. In terms of its length, thickness, orientation,and other characteristics, the first arm should be configured so that aforce applied to its distal end will transmit a torque to the cranksufficient to cause the crank to rotate. Typically, the force can beproduced directly or indirectly by the subject employing the device.

The amount of force required to rotate an unencumbered crank of givensize and mass will depend at least in part on the length of the firstarm. While it should be apparent that the first arm can have manypossible lengths in accordance with this invention, the minimum lengthshould at least be such that a force generated by a human subject wouldresult in rotation of an unencumbered crank. The length of the first armcan preferably be such as to allow an adult human subject to keep one ofhis or her limbs in contact with the distal end of the arm throughout afull rotation of the crank. In accordance with these considerations, thefirst arm can generally be from 2 inches to 36 inches, more particularlyfrom 3 inches to 24 inches, and preferably from 4 inches to 10 inches inlength.

The distal end of the first arm can be shaped in a way that facilitatesthe application of force to the arm by the subject. This can beaccomplished by a right-angle bend of the distal end of the arm itself,or by an attachment to the arm. One example of an attachment is afoot-pedal such as is found on the crank of a conventional stationarybicycle. Those having skill in the art will recognize that any extensionwhich extends outward substantially perpendicular to the body of thefirst arm and for a sufficient distance to allow a subject to grasp orstep thereon can be utilized with the present invention. Additionalcomponents that facilitate the type of exercise involved here, such asone or more straps or commercial pedal binding systems to secure thelimb to the first arm can also be used. Such straps or retainers can bebeneficial to guide the limb along with the first arm. However, suchstraps are not generally intended to be used for forced upward motions.Specifically, the assist mechanism is designed to provide supplementalforce sufficient to drive the first arm upward without substantialupward force using the associated limb.

The device can also comprise a second arm that is connected to the crankand extends substantially perpendicular to the axis of rotation and in asubstantially opposite direction from that of the first arm. Like thefirst arm, the second arm is configured to communicate torque to thecrank and cause the crank to rotate. Therefore, according to oneembodiment, the second arm can be essentially identical in shape anddimension to the first arm. However, the force applied to the second armis supplied by an assist mechanism rather than the subject. The shapeand dimension of the second arm for a given embodiment can depend on thenature of the assist mechanism used in that embodiment.

Devices in accordance with the present invention can include an assistmechanism by which a subject is allowed to exercise a single limb usinga cyclical motion and forces which are similar to the motion used withpaired-limb ergometers. One unique characteristic of the devicedisclosed here is that the subject need only exert himself for a portionof the cyclical motion. For example, a human subject using two legs in apedaling motion on a paired-limb ergometer need only actively extendeach leg in an alternating fashion to accomplish continuous rotation ofthe crank. The force needed to complete the flexion portion of themotion for each leg can be supplied by the extension of the oppositeleg, so that flexion is primarily passive. However, because oxygen ispreferably delivered at high rate to both legs, the potential gain inaerobic capacity for each leg is less than if exercised one at a time.If the subject used only one leg on a paired-limb ergometer, he wouldneed to both actively extend the limb and flex the limb to accomplishcontinuous rotation. The muscles used to flex the leg are typically moreeasily fatigued in humans, so this manner of use would result in rapidfatigue and therefore less benefit. In contrast, with the deviceprovided by the present invention, the force needed to complete theflexion portion of the motion for the exercising leg is supplied by theassist mechanism. Therefore, the subject can gain the benefit oftwo-legged cycling, i.e. exercising for a longer period of time withless fatigue, while also reaping the benefit of single-leg cycling, i.e.a higher rate of oxygen delivery to the exercising leg.

In one embodiment of the present invention, the assist mechanism is acounterweight that is connected to the second arm. The counterweight canassist in rotating the crank during a portion of the rotation cycle,e.g. during limb flexion, by virtue of the momentum imparted to thecounterweight during another portion of the rotation cycle, e.g. limbextension, and also by the effect of gravity. Therefore, it should berecognized that the minimum weight of the counterweight should be enoughso as to achieve the momentum needed to rotate the crank. Those havingskill in the art will be aware that the weight needed will depend on anumber of factors, such as the force applied to the first arm by thesubject, the length of the second arm, the radius of the crank, and thestrength of the subject. In general, the counterweight can weigh fromabout 5 pounds to about 70 pounds. More particularly, it can weigh fromabout 10 pounds to about 50 pounds. According to a still more particularembodiment, the counterweight can weigh from about 20 pounds to about 40pounds.

According to one embodiment, the counterweight can be constructed from asingle solid piece of material, such as iron, steel, lead, or otherdense material. In an alternative embodiment, the counterweight can be ahollow container with an opening through which it can be filled. Thecounterweight could be given a desired weight by filling it with asufficient amount of filler material. Examples of suitable fillermaterial can be water, sand, gravel, or steel pellets.

The counterweight can be configured to attach to the second arm by anyof a number of mechanisms, either permanent or temporary. In oneembodiment, the counterweight can be attached by an interferencefitting. In an alternative embodiment, the counterweight can beconfigured to attach by a threaded fastener. In another alternativeembodiment the counterweight can be attached to the second arm by aclamp. Using a removable attachment has the advantage of allowing asingle device to be usable with a limb on either side of a subject'sbody. To switch from exercising one limb to exercising the opposite limbsimply involves replacing the counterweight from one arm with the pedal(or equivalent component) from the other arm, and vice versa. Thisapproach would be most suited to embodiments where the first arm andsecond arm are essentially identical.

Other components or mechanical arrangements can exist that serve as asecondary assist mechanism. A secondary assist mechanism can comprise apneumatic system, where the force supplied to the crank during a portionof its cycle comes from the expansion of a compressed gas. In a moreparticular aspect, the compression of the gas can be accomplished by aforce supplied by the subject. Alternatively, the assisting force can besupplied by a spring mechanism, where the spring is tensioned duringpart of the rotation cycle and the subsequent release of that tensiondrives a portion of the rotation cycle. Still another alternative caninvolve a motor operatively connected to the crank and configured toprovide torque thereto during the appropriate portion of the rotationcycle. The present invention is intended to encompass any such variationthat serves to assist in a portion of the crank rotation cycle so as toallow an efficient mode of exercise such as described above.

In a particular embodiment, the device can further comprise a brakeoperatively connected to the crank and configured to resist the rotationof the crank. The brake can be used to prevent the crank from turning,and thereby securing the device for periods of nonuse. Alternatively,the brake can be used only to make it more difficult to rotate thecrank, thereby providing a resistance mechanism to increase theintensity of exercise. Such a brake can operate via surface-to-surfacefriction by pressing a friction material against the crank or flywheelto generate the friction. This can be accomplished using calipers or abrake pad for example, where the braking surface is covered with rubberor other composite material. It is also possible that the assistmechanism can function as a brake in accordance with the presentinvention. Typically, this will involve utilizing inertia to generatefriction, such as the inertia present in the components of a motor or ina counterweight. A number of braking mechanisms for ergometers andmoving bicycles are known in the art, and one with skill in the art willrecognize those that can be used with the present invention. Additionalnon-limiting examples of suitable braking mechanisms can includemagnetic, electrical, fluid, and aerodynamic resistance mechanisms.

A typical paired-limb ergometer can be modified to operate as asingle-limb ergometer in accordance with this invention. To do soinvolves removing one pedal or crank arm from the ergometer andreplacing it with a counterweight having a suitable connector. Thereforethis invention also provides a retrofit kit for use in making such amodification. Such a kit can comprise a counterweight and a fastenerconfigured for attaching the counterweight to an arm of an ergometer.Preferably the fastener used will allow the counterweight to be removedand remounted as needed. As an additional aspect, the counterweight canbe isolated, positioned and shaped so as to prevent striking the subjectduring use. More particularly, a safety wall or other type of shield maybe situated so as to isolate the counterweight from the limbs of thesubject, allowing the counterweight to freely rotate during use whilepreventing it from striking the subject. Further, such retrofit kits canalso include written installation instructions corresponding to aparticular model or configuration of exercise device to be modified. Thewritten instructions can include directions tailored for a particularinstallation, depending on the type of fastening used. Optionally, theretrofit kit can include a suitable tool which can be used to remove anexisting pedal and/or install the counterweight. Non-limiting examplesof a suitable tool can include allen wrench, fixed wrench, screwdriver,and the like.

A common setup used for stationary cycling exercise involves the use ofa bicycle trainer. This approach is often used to facilitate stationaryexercise on a road bicycle, thereby providing a riding experience thatmore closely resembles actual road riding than is possible withconventional stationary exercise bicycles. Bicycle trainers usuallycomprise a stationery structure upon which a bicycle may be mounted andmay have various configurations depending on the desired features andtype of bicycle. For example, a road bicycle may be mounted so that therear wheel of the bicycle is in contact with the trainer rather than theground. Alternatively, the rim of the rear wheel can be contacted as inthe case of many mountain bike trainers. Trainers typically featuremechanisms to apply resistance to the rear wheel of the bicycle, so asto simulate riding under different conditions, such as climbing a hill.A retrofit kit as described above may be used to modify a bicycle foruse on a bicycle trainer, so that the benefits of single-leg cycling maybe enjoyed in combination with the benefits arising from bicycletrainers.

As discussed above, exercise bicycles constitute one form of ergometer,but can also include free road bicycles. Accordingly, one aspect of thepresent invention provides an exercise bicycle. Such a bicycle comprisesa frame and a crank to which at least two arms are connected, where afirst arm transmits to the crank an external force, such as can beapplied to the device by the subject, while a second arm transmits tothe crank a force supplied from an assist mechanism. The frame may beany structure configured to support a subject using the bicycle, as wellas providing attachment points for apparatuses that facilitate use ofthe device. Such apparatuses include conventional bicycle parts, such asa seat on which a subject may sit or recline, and handlebars or anequivalent structure to serve as a handhold and to provide support tothe subject's upper body. As discussed above, the crank of such abicycle is caused to rotate by forces applied to the arms connectedthereto, where the subject exerts a downward force to the first arm byextension of one leg while an assist mechanism provides a force to thesecond arm. In a preferred embodiment, the crank and arms are configuredas a pedal assembly. The assist mechanism is preferably a counterweightthat is operatively connected to the second arm. In general, thecounterweight can weigh from about 5 pounds to about 70 pounds. Moreparticularly, it can weigh from about 10 pounds to about 50 pounds.According to a still more particular embodiment, the counterweight canweigh from about 20 pounds to about 40 pounds. The exact weight may varydepending on the particular user and desired exercise conditions, butshould generally be sufficient to affect a power curve during use whichsubstantially corresponds to non-assisted motion at the same rotationalvelocities. Thus, the return assist mechanism of the present inventionis a passive assist mechanism rather than an active assist mechanismsuch as electric motors, hydraulic arms, or the like.

A particular embodiment of the exercise bicycle further comprises a footrest configured such that a subject using the bicycle may place one footupon the foot rest while maintaining contact between the other foot andthe first arm through a complete rotation of the crank. In one aspect ofthis embodiment, the foot rest includes a retaining mechanism to allowthe foot to be secured to the foot rest. Any suitable retainingmechanism can be used in connection with the present invention. Theseinclude structures into which a foot may be inserted, such as straps,stirrups, toe clips, or the like. Such systems have the advantage of notrequiring a subject to wear specialized footwear. Alternatively, otherstructures may be used, such as clipless (e.g. commercial pedal bindingsystems) pedal systems in which a fastener is designed to positivelyconnect with a complementary structure on the sole of a specializedshoe. A foot rest may also be part of the retrofit kits described above.In this case, the foot rest may be configured to attach to the frame ofa road bicycle or exercise bicycle. One important aspect of a foot restis to keep the foot and leg safely out of the way of the moving counterweight.

The present invention also provides methods for exercising themusculature associated with a single limb that can impart greater gainsin muscle strength and aerobic capacity than analogous paired-limbexercise. In one embodiment, a method of exercise can include applying asubstantially downward force to an arm that is operatively connected toa crank, such as a first arm as described above, so that the crankrotates accompanied by downward movement of the arm. This movementcorresponds to the extension of the limb used to apply the force. Thecrank is then allowed to continue rotating, carrying the arm (andtypically the extremity of the limb) eventually upward. This isaccompanied by flexion of the limb; however, this portion of therotation cycle and the resulting flexion are assisted by an assistmechanism such as those discussed above. In a particular embodiment, theassist mechanism is a counterweight. As a result, little or no externalforce (such as from flexor muscles of the limb) is needed to effect thisportion of the rotation cycle. Accordingly, the methods and devices ofthe present invention allow for dramatically improved biomechanicalmotion which approximates forces experienced under normal paired cyclingwhile reserving oxygen capacity for a single limb. Typically, anassisted power curve during cycling of the device substantiallycorresponds to a non-assisted power curve when each is measured at acommon rotational velocity. FIG. 1 (explained in more detail below) is acommon example of this correspondence between assisted and non-assistedmotion. Preferably, deviation at maximum extension and deviation atmaximum flexion between the assisted power curve and the non-assistedpower curve are each less than about 75%, and preferably less than about50%. Most preferably, the deviation at maximum extension is less than10%. “Deviation” is defined as the percent difference between thenon-assisted power and the assisted power divided by the non-assistedpower. Similarly, CO₂ consumption for single limb exercise can besubstantially identical to paired-limb exercise at the same power for aparticular subject. Typically, assisted CO₂ consumption can deviate fromnon-assisted CO₂ consumption by less than 20%, and preferably less thanabout 10%.

The methods of the present invention, particularly embodiments whichcomprise application of substantially downward forces by a subject, areespecially suited to exercising the human leg, where the subject ispositioned higher than the crank. However, it should be understood thatthe method can also be used where the subject is positioned differently,e.g. prostrate, recumbent, or supine, and also for exercising other bodyparts such as an arm. For those modes of exercise, the directionality ofapplied force and assisting force can differ yet remain in accordancewith the present invention.

The following example illustrates one embodiment in accordance with thepresent invention. However, it is to be understood that the following isonly exemplary or illustrative of the application of the principles ofthe present invention. Numerous modifications and alternativecompositions, methods, and systems can be devised by those skilled inthe art without departing from the spirit and scope of the presentinvention. The appended claims are intended to cover such modificationsand arrangements. Thus, while the present invention has been describedabove with particularity, the following Examples provide further detailin connection with several specific embodiments of the invention.

EXAMPLES

A comparison was made of force and metabolic data measurements takenfrom a human subject when using (a) a conventional exercise bicycle, and(b) an exercise bicycle device designed for single-leg exerciseaccording to the present invention. The single-leg exercise device wasequipped with a 30 lb. counterweight on the pedal on the side oppositethe leg being exercised (termed the “off-leg” side). As shown in FIG. 1,the power and associated forces on the on-leg pedal throughout the pedalcycle were substantially similar on the conventional exercise bicycleand the single-leg exercise bicycle. This indicates that, in single-legpedaling, the counterweight contributed to pedal rotation very much likethe subject's off-leg did in conventional paired-leg pedaling. Oneimportant result of such performance is that it provides for passiveflexion of the exercised leg while still producing normal pedalrotation. This is important in avoiding excessive fatigue and can allowfor extended exercise of specific muscles. In contrast, when single legcycling was performed without the counterweight, power was reducedduring the extension phase and increased during the flexion phase. Thus,single leg cycling without a counterweight is biomechanically differentthan normal two-legged cycling, whereas single leg cycling with acounterweight in accordance with the present invention is quite similarto normal two-legged cycling.

FIG. 2 shows the metabolic rate measurements taken from the subjectwhile pedaling each of the two devices. As the figure shows, oxygenconsumption was linearly related to power output for single anddouble-leg cycling. Further, the oxygen uptake for single leg cyclinglies on the regression line from the double-leg data. Thus, thecounterweighted device allows steady state submaximal cycling, to beperformed with the similar efficiency and metabolic cost as normal twolegged-cycling.

While the forgoing examples are illustrative of the principles of thepresent invention in one or more particular applications, it will beapparent to those of ordinary skill in the art that numerousmodifications in form, usage and details of implementation can be madewithout the exercise of inventive faculty, and without departing fromthe principles and concepts of the invention. Accordingly, it is notintended that the invention be limited, except as by the claims setforth below.

It is to be understood that the above detailed description is onlyillustrative of the application for the principles of the presentinvention. Numerous modifications and alternative arrangements can bedevised without departing from the spirit and scope of the presentinvention. While the present invention has been shown in the drawingsand fully described above with particularity and detail in connectionwith what is presently deemed to be the most practical and preferredembodiment(s) of the invention, it will be apparent to those of ordinaryskill in the art that numerous modifications can be made withoutdeparting from the principles and concepts of the invention as set forthherein.

1. A muscle exercise device, comprising: (a) a crank having an axis ofrotation; (b) a first arm connected to the crank and extendingsubstantially perpendicular from the axis of rotation, configured sothat a downward external force applied to the distal end of the firstarm causes rotation of the crank; (c) a second arm connected to thecrank and extending substantially perpendicular from the axis ofrotation in a direction opposite to that of the first arm; and (d) acounterweight operatively connected to the second arm and weighing from5 to 70 pounds.
 2. The device as in claim 1, further comprising a brakeoperatively connected to the crank and configured to resist rotation ofthe crank.
 3. The device as in claim 1, wherein the counterweightcomprises lead.
 4. The device as in claim 1, wherein the counterweightcomprises iron.
 5. The device as in claim 1, wherein the counterweightcomprises a hollow container having an interior and at least onesealable opening providing access to the interior.
 6. The device as inclaim 1, wherein the counterweight has a weight of about 20 to 40pounds.
 7. The device as in claim 1, wherein the counterweight isconnected to the second arm by a threaded fastener.
 8. The device as inclaim 1, wherein the counterweight is connected to the second arm by aninterference fit.
 9. The device as in claim 1, wherein the counterweightis connected to the second arm by a clamp.
 10. The device as in claim 1,wherein the muscle exercise device is an ergometer, bicycle trainer, orexercise bicycle.
 11. The device as in claim 1, further comprising asecondary assist mechanism operatively coupled to the crank.
 12. Thedevice as in claim 1, further comprising a frame retaining the crank ina fixed position and including a seat coupled thereto in a positionsuitable for use in extension and flexion motion of legs of a subject.13. The device as in claim 1, further comprising a foot rest attachedthereto, said foot rest having an upper surface and a lower surface, andbeing configured so that a first foot of an exercising subject can reston the upper surface while a second foot of the subject is in operativecontact with the first arm.
 14. The device as in claim 1, furthercomprising a stationary shield situated lateral to the counterweight andconfigured to prevent the counterweight from striking an exercisingsubject.
 15. A retrofit kit, comprising: (a) a counterweight; and (b) afastener configured for attaching the counterweight to an ergometer orbicycle trainer crank arm.
 16. The retrofit kit of claim 15, furthercomprising a set of written installation instructions corresponding to aparticular configuration of the ergometer crank arm.
 17. The retrofitkit of claim 15, further comprising a tool for removing an existingpedal and/or securing the fastener.
 18. A method of exercisingmusculature associated with a single limb, comprising the steps of: (a)applying a downward force to an arm operatively connected to a crank,said force being sufficient to rotate the crank so that the arm movesfrom an upper position to a lower position; and (b) return assistrotating the crank sufficient to continue rotating such that the armreturns to the upper position, wherein the return assist rotating isassisted by a counterweight weighing from 5 to 70 pounds.
 19. The methodof claim 18, wherein an assisted power curve during the steps ofapplying and return assist rotating substantially corresponds to anon-assisted power curve each measured at a common rotational velocity.20. The method of claim 19, wherein a deviation at maximum extension anda deviation at maximum flexion between the assisted power curve and thenon-assisted power curve are each less than about 75%.
 21. The method ofclaim 20, wherein the deviation at maximum extension is less than 10%.22. The method of claim 18, wherein a CO₂ consumption for single limbexercise is within 20% deviation of a CO₂ consumption for paired-limbexercise at a common power per subject.